This invention relates generally to a multiphase load control system that provides phase voltages to a connected load via split voltage sources which are connected together or maintained disconnected through switches that are controlled to vary the application of the phase voltages to the load. In a particular aspect of the present invention the phase voltages, which are provided to a connected load when the switches connect the split sources, are greater than the voltages on the switch devices; therefore, the switch devices can have voltage ratings less than the phase voltages.
For proper operation of electrical systems, as with mechanical systems, components must be strong enough to withstand forces which will exist in the systems in which they are used. For example, an electrical component must be able to accommodate the electrical "pressure" (i.e., voltage) that can be applied to it within a particular system in which it is used. In at least alternating current (ac) systems, one measure of the "strength" of a component is indicated by its "voltage rating."
With regard to this voltage characteristic, a simple way to design an electrical system is to determine the maximum voltage and then select components which have voltage ratings at least as large as the maximum voltage. This, however, can be expensive. A conventional submersible pump system illustrates this.
In a submersible pump system, such as used in a well, a pump and a motor for driving the pump are located in the well. When the motor is a three-phase motor, three conductors are connected to the motor and extend out of the well to a motor control system at the surface. The motor control system provides a three-phase voltage through three output lines to which the conductors running into the well are connected. In one such motor control system, a respective switch is connected in electrical series in each output line. In this example, these switches are part of a device called a "contactor" which has "poles" implemented with relay coils and switch members; in this case, one three-pole contactor is used so that each pole is in a respective output line. These switches are closed and opened to control the application of voltage to the motor, thereby controlling whether the motor is on or off and thus also controlling the pump driven by the motor.
In the foregoing example, the contactor device needs to be rated for the full phase voltage that can be applied to the motor. Such a contactor device can be relatively expensive. For example, today's conventional contactor devices which have voltage ratings greater than 600 volts(RMS) are typically significantly more expensive than contactor devices rated for 600 volts or lower. Unfortunately, however, the phase voltages in many applications of the foregoing example are greater than 600 volts so that the more expensive contactor device rated above 600 volts is needed. It would be financially advantageous if a relatively high voltage system as described above could be implemented with lower voltage rated components, specifically the less expensive 600-volt rated contactor device.